Non-invasive ultrasonic pulse doppler cardiac output monitor

ABSTRACT

A cardiac output measurement device for the real-time, non-invasive measurement of cardiac output that can be effectively operated by relatively unskilled personnel on a routine monitoring basis in a wide variety of office and hospital conditions. To accomplish this task, the system utilizes a pulsed Doppler ultrasound transducer directed through the suprasternal notch of a patient axially towards the blood flow in the ascending aorta. The device automatically searches the ascending aorta at various predetermined depths to find the depth at which the greatest quality blood velocity reading is detected. An examination is performed at that chosen depth and the device automatically calculates a patient&#39;s cardiac output from the Doppler measured velocity combined with an aortic diameter estimation made from the patient&#39;s height, weight, and age. The device automatically calculates cardiac velocity, cardiac index, stroke distance, heart rate, and stroke volume.

RELATED APPLICATIONS

This application is a continuation-in-part cf U.S. patent applicationSer. No. 120,882 filed on Nov. 16, 1987, now abandoned.

FIELD OF THE INVENTION

The invention relates to a device for the non invasive measurement ofcardiac output, and more specifically to a device for the non-invasivemeasurement of cardiac output of a human by means of a pulse-Dopplerinsonification technique.

BACKGROUND OF THE INVENTION

It is particularly desirable at times to measure the cardiac output of apatient on a real time basis without employing invasive surgicaltechniques. Non-invasive measurement of cardiac output using Dopplerultrasound has been a goal for many years. Success has been reportedusing duplex imaging equipment by combining either echographic or M-modemeasurement of aortic diameter as measured from the second or thirdintervostal space with Doppler velocity measurements performed from thesuprasternal notch. Good correlation with thermodilution cardiac outputmeasurements have been reported using these techniques. The primarydrawback of these techniques is the requirement for relatively expensiveequipment and highly skilled operators to perform the measurement.

Several previous approaches to making a dedicated instrument to computecardiac output have used continuous wave Doppler from the suprasternalnotch of a patient. These techniques relied upon CW Doppler to measureblood flow velocities in the ascending aorta, the aortic arch, or thedescending aorta. Although these implementations proved quite successfulin the hands of skilled operators, routine clinical application was mademore difficult by the potential confusion of signals that may occur withCW Doppler. When measurements are attempted for flow in the ascendingaorta, it is not uncommon to also find flow signals in the same areafrom the innominate artery on the right or the left carotid orsubclavial arteries to the left. Although a window is generallyavailable in which only the aortic signal can be found, an unskilledoperator may have difficulty determining the difference between thesesignals from the aorta and other such signals available from theinnominate carotid, or subclavian arteries. Flow signals measured fromthe aortic arch or descending aorta are potentially less representativeof total cardiac output due to Doppler angle, as well as the lack ofknowledge of the unknown percentage of flow which has been directed tothe head. Flow measurements in the descending aorta may provide a goodtrend indicator but cannot readily provide absolute cardiac outputinformation.

In the past, one method used to measure the cardiac output of a patientrequired a doctor to anesthetize a patient and insert an ultrasonictransducer probe in the esophagus near the aorta of the heart with muchdiscomfort to the patient. Another method of cardiac output measurementinvolved the surgical insertion of a detector in the pulmonary artery ofa patient. Use of this method was generally limited to extremely illpatients because it is a particularly risky operation.

Since continuous wave (CW) Doppler devices do not provide any rangediscrimination, no means is available for limiting the range distancefrom the transducer along the direction of the ultrasonic beam so thatthe measurement being taken may be optimized to correspond where thebest reading for cardiac output velocity should be taken. This "best"location corresponds to a point approximately 2 cm above the annulus ofthe aortic valve at which point any turbulence associated with a normalaortic valve has diminished and a more or less uniform flow profileexists across the area of the aorta. It is not possible, when usingcontinuous wave insonification, to uniquely determine the location alongthe ultrasonic beam from which a returning energy wave was reflected, todetermine precisely where the reading came from, and if the readingrepresents blood flow in the ascending aorta. These systems require anultrasound technician or a cardiologist to accurately analyze the outputof the device. U.S. Pat. No. 4,509,526 discloses a device that usescontinuous wave insonification to measure the blood flow velocity in theascending aorta of a patient which is combined with a separatemeasurement of aortic diameter to provide a measure of cardiac output.This device requires a highly skilled ultrasound technician or acardiologist to operate and to interpret the tracer display to receive avalid cardiac output velocity reading. The system does not have theability to detect aortic velocity selectively at different distancesfrom the transducer due to the fact that it uses continuous waveultrasound. The device also does not have the capability to distinguishbetween signals representing noise or other reflected signals and

signals representing blood flow in the ascending aorta.

SUMMARY OF THE INVENTION

The invention relates to a method and device for the automaticmeasurement of blood velocity in the ascending aorta utilizing pulseDoppler ultrasound and having the capability of automatically searchingfor the optimal measurement of blood velocity at different depths in theascending aorta of a patient. The cardiac output monitor of theinvention interacts with the user through a visual display, a printer,and a keyboard. The user may input various parameters and data about thepatient through the keyboard. Values corresponding to a patient'sheight, weight, and age are input into the device, from which a valuefor the patient's aortic cross sectional diameter is estimated using anestablished formula. Alternatively, a value for aortic cross-sectionaldiameter obtained from another method may be input into the device andused to calculate stroke volume, cardiac output, etc.

The device performs multiple signal processing tasks simultaneously inreal time. The received RF signals are demodulated to provide Dopplershift signals in the audio frequency range which correspond to bloodflow velocities. The Doppler shift signals are analyzed in the frequencydomain to determine a modal or average Doppler shift frequency for eachinstant in time. A visual bargraph of these modal or average frequenciesis displayed on the front of the instrument. A time series of the modalor average frequencies hereafter designated AVP (aortic velocityprofile) is formed and processed to yield such parameters as heart rate,stroke distance, maximum velocity, and other pertinent statisticalparameters including Karhunen-Loeve expansion coefficients used inpattern recognition algorithms.

The device emits a real time audio signal, proportional to blood flowvelocity detected in the ascending aorta of a patient. The operator canuse this signal, as well as the visual display, to position atransducer, thereby obtaining a Doppler shift signal from the ascendingaorta. A sophisticated signal pattern recognition system is employed inthe device to analyze the time series of modal or average frequenciesand check it against several criteria before it is confirmed to be atime series corresponding to flow within the ascending aorta. If thetime series does not pass the criterion, it is rejected and the operatormust reposition the transducer probe to find a better signal.

The sampling and pattern recognition are performed continuously to tryto obtain signals which are within the established range of patternrecognition criteria. The device will automatically search for bloodflow velocity at different depths in the aorta and will compute anaverage maximum value for blood flow velocity at each of the depthssearched. The depth at which the device detects the greatest averagepeak systolic blood flow velocity is the depth that is chosen by thedevice to perform the examination. As soon as a sufficient number ofsignals pass the pattern recognition criteria at this depth, theexamination is automatically terminated and the results are presented tothe operator on the visual display, or the printer if chosen.

The transducer probe of the device is designed to fit within thesuprasternal notch of a patient with minimal discomfort to insonify aregion of the ascending aorta to detect blood flow velocity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the device of the invention;

FIG. 2 is a schematic drawing of the device of the invention; and

FIG. 3 is an elevation view of the front panel of the device of theinvention;

FIG. 4 is an elevation view of the back panel of the device of theinvention;

FIG. 5 is a schematic diagram of a data input sequence used in thedevice of the invention;

FIG. 6 is a schematic diagram of "Acquisition" mode utilized in thedevice of the invention; and

FIG. 7 is a schematic diagram of "Acquisition Calculation" mode utilizedin the device of the invention.

FIG. 8 is a bottom view of the transducer probe utilized in the deviceof the invention;

FIG. 9 is an elevation view of the transducer probe utilized in thedevice of the invention;

FIG. 10 is a schematic diagram of a portion of the real time patternrecognition mode utilized in the device of the invention;

FIG. 10A is a schematic diagram of another portion of the real timepattern recognition mode utilized in the device of the invention;

FIG. 11 is a graph showing an average aortic velocity profile;

FIG. 12 is a bar graph showing relative signal class energycorresponding to the eigenvectors of FIG. 13; and

FIG. 13 shows a plurality of graphs showing the eigenvectorscorresponding to the eigenvalues of FIG. 12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS General Approach

In general, use of pulse Doppler ultrasound in a "blind" search mode isconsidered more difficult without the aid of an accompanying image. Thedevice of the invention operates in a "blind" search mode. To get aroundthis difficulty, the system has a series of range gates provided atdepths certain to be beyond the innominate artery in order to measureflow in the ascending aorta. An initial depth of range gate is selectedby the instrument based upon the subect's height. This initial searchdepth is used to locate the flow in the ascending aorta.

The device provides the operator with information about blood velocityin the ascending aorta so that the optimal location for measurement ofblood velocity may be found. The user manipulates the transducer in thesuprasternal notch of the patient and the Doppler range gate isautomatically stepped through a search from 7 to 10 cm. During this"search in depth", the device performs real time pattern recognition ofthe received signals, and saves those signals which pass the patternrecognition criteria. Once the optimum depth is determined, the cardiacoutput and heart rate, as determined at that depth, are displayed. Thedevice then returns to that depth and acquires data for a repeatmeasurement of cardiac output. Typically, the entire examination takes 1to 2 minutes to perform and may be easily repeated with minimaldiscomfort to the patient.

Several assumptions underlie the method the device uses for computingblood velocity. These assumptions are common to all of the Dopplermeasurement techniques described above.

The first assumption is that the angle between the Doppler beam and thedirection of flow in the ascending aorta is zero degrees. The velocityof blood flow detected by Doppler ultrasound techniques is directlyproportional to the Doppler shift frequency. In computing blood flowvelocity using Doppler ultrasound, the only angular dependence is acosine factor of the angle between the Doppler beam and the direction ofblood flow being measured. The cosine factor is not considered asignificant limitation or source of error because a moderate angleproduces a very small error in the velocity calculation.

The second major assumption is that systolic flow in the ascending aortacan be characterized as plug flow. Plug flow assumes a blunt flowprofile across the aortic flow diameter. The assumption of plug flowduring systole in the ascending aorta appears well justified with someexceptions. The flow stream diameter is generally stated to beequivalent to the diameter of the aortic valve which is typically equalto the aortic diameter just above the sinus under normal circumstances.One exception occurs when patients have aortic stenosis. In thisinstance, the flow diameter may be significantly less than the flowdiameter of the ascending aorta. Therefore, use of this instrument onpatients with aortic stenosis is not valid unless the orifice diameteris known. Another condition which would compromise the cardiac outputresult of this device would be aortic insufficiency in which case theregurgitant flow fraction of blood would be unknown. In addition, anyother condition which might produce extreme turbulence in the ascendingaorta could compromise the results.

In order to compute volume flow of blood, the cross sectional area ofthe ascending aorta, the heart rate, and the flow velocity integral, orstroke distance, must be determined. The stroke distance is defined asthe area under the mean velocity profile per beat. The stroke distancemultiplied by the cross-sectional area produces the stroke volume andthe stroke volume multiplied by heart rate produces the cardiac outputmeasurement.

A key factor in making a reliable and easy-to-use instrument is toprovide a device capable of making almost instantaneous automatedcalculations to steer the user to the correct signals as well asnotifying the user of signal characteristics indicative of patientconditions which would make the measurement unreliable. Patternrecognition techniques are employed to analyze the time series modalfrequency waveforms in order to determine the efficacy of any individualmeasurement. The term "pattern recognition" simply implies that signalcharacteristics are computed and compared to existing criteria to chooseonly those signals which are within prescribed limits for acceptablemeasurement. To perform pattern recognition, a set of conditions must bedescribed which can be used to define the difference between usable andunusable signals.

Characteristically, flow in the ascending aorta during systole istowards the transducer placed in the suprasternal notch. Lack of flowduring diastole is also a characteristic. Occurrence of significantprolonged flow in the reverse direction away from the transducer is anindication of abnormal aortic flow or flow in a vessel other than theaorta. These and other parameters such as movement of a ventilator,breathing and noise can be used to determine the acceptability of anygiven signal based upon knowledge of what is normal or reasonable forflow in the ascending aorta.

Measurement of the aortic diameter can be problematic. It can generallybe determined echocardiographically or from M mode measurements;however, this requires the use of relatively expensive equipment. It hasbeen known for some time that aortic diameter typically increases withage. Aortic diameter also tends to increase with body surface area.Therefore, an aortic diameter estimate was developed based upon theheight, weight, and age of the subject.

The following detailed description is intended to typify a preferredembodiment of the invention. Those of ordinary skill in the art willreadily recognize that alterations and variations to many details of thesystem may be made as desired.

Functional Description

The Doppler is activated with automatic gain control at a gate depth ofseven or eight centimeters depending on the height of the patient.Generally, if a patient is less than 5'6" tall, the search will begin at7 cm depth. If the patient is 5'6" tall or greater, the search willbegin at 8 cm because the 7 cm depth would most likely be too distantfrom the heart to yield a good reading.

Referring to the schematic drawing of FIG. 2, the Doppler unit (2)transmits a ten-cycle tone burst at 2.4 MHz with PRF of 8.0 kHz andmaximum peak-to-peak voltage of 50 Volts to the transducer (1). Withineach PRF, a time delay is performed before sampling the Doppler-shiftedwave form, resulting in gate depths of 7, 8, 9, or 10 cm. Both the gainof the receiver and the gate depth are controlled by the microprocessor(8).

The Doppler unit detects the in-phase and quadrature signals at theselected gate depth, and these signals are passed to the audio signaloutput (4) and the 12 bit analog-to digital (a/d) converter (3) The twoa/d values are multiplexed to a 1 kByte buffer (5) which forms the timeseries input to the digital signal processor (6).

At the end of each group of 64 PRF's, or 8.0 ms, the digital signalprocessor reads a section of the time series data and performs a fastfourier transform (FFT). Features are extracted from the resultingspectrum, stored in a 16 kbyte RAM (7), and passed to the microprocessor(8).

The microprocessor (8) executes instructions loaded from EPROM (9) whichstores 64 kbytes of programs and data tables. Information can beselectively stored in a RAM (10) and utilized by the microprocessor. Themicroprocessor performs pattern recognition upon features extracted bythe digital signal processor. In addition, it performs system levelcontrol of the device. There are three real-time tasks of themicroprocessor: determination of optimal gain, determination of optimaldepth of the aortic velocity profile, and maintenance of signal qualityduring data acquisition at optimal gain and depth.

After twelve systoles are recognized by the microprocessor, the Doppleris turned off and the following values are calculated: cardiac output,cardiac index, stroke volume, stroke index, heart rate, stroke distance,maximum velocity, acceleration, and ejection time.

The full examination results can be optionally sent to the printer (13)and are stored in non-volatile memory (12). A tone generator (14) may beincluded to signal a user of the device as to predetermined systemconditions.

System Hardware

The cardiac output monitor of the invention, in a preferred embodimentshown in FIGS. 1, 3 and 4, comprises a generally rectangular housing(20) having a front panel (21), back panel (40), and a plurality of sidepanels. The front panel (21) contains the interactive interfaces used bythe operator to input information to the device and to receiveinformation and printouts from the device.

An interactive visual display (24) is carried by the front panel (21) todisplay information pertinent to the examination. The visual display(24) preferably comprises a twenty space LED read-out for displayinginformation and to guide the operator through a patient examination. Theinteractive visual display (24) utilizes an alphanumeric single linedisplay to relay complex information obtained in the process of theexamination and to display a continuous bargraph corresponding to bloodflow detected by the transducer probe (31).

A volume control (25) is carried by the front panel (21) enabling theoperator to adjust the volume of an audio signal output to an audiospeaker, designated as (4) in FIG. 2, which may be at any desiredlocation on the monitor, the signal corresponding to blood velocity.

A printer drawer (26) may open from the front panel (21) of the cardiacoutput monitor and preferably contains a printer (shown as (13) in FIG.2) to print the results of the examination. The printer drawer (26)desirably includes a forwardly facing panel upon which is carried anexit port (32) for the printer paper (28) enabling the operator toreceive a hard copy printout of the examination without having to openthe printer drawer (26). A paper advance button (27) is convenientlylocated on the forwardly facing printer drawer panel (26) enabling theoperator to advance the paper (28) so that it may be removed from thedevice without damaging the readability of the paper.

The front panel (21) of the cardiac output monitor also includes afinger operated keypad (22) through which information is input to thesystem and the examination is controlled. The key pad (22) comprises atleast 12 keys arranged in a grid pattern with ten keys representing thenumbers 1,2,3,4,5,6,7,8,9,0, one key representing start/yes, and anotherkey representing stop/no. In order to enter data into the device theoperator must press the appropriate keys on the key pad (22) in responseto prompts appearing on the visual display (24).

A probe storage drawer (23) is carried by the front panel (21) providinga convenient and protective storage space for the probe (31) when it isnot in use. The storage drawer (23) includes a cover which may have asmall opening (33) along an edge through which the wire (29) attached tothe transducer probe (31) may protrude outwardly of the storage drawer(23) while the drawer is in a closed position. The probe wire (29) maybe connected to the device by means of an outlet jack carried by one ofthe interior walls of the probe storage drawer (23). The probe wire (29)may include an activation switch (30) to control the probe (31).

Carried by the back panel (40) of the device, as shown in FIG. 4, theon/off power switch (41) is used to enable and disable electric power tothe device. This switch (41) may comprise a simple on/off toggle switchor the like.

An external speaker output jack (42) is provided on the back panel (40)so that an external speaker may be connected to the device enabling theaudio output to be heard at another location. The external speaker maycomprise a pair of headphones enabling the operator of the device tohear the audio signal with minimal distraction. Circuit breaker resetbuttons (43) are carried by the back panel (40) to enable the operatorto reset the circuits if they should become disabled, and one or moreexternal grounding terminals (44) are carried by the back panel (40) toenable the device to be connected to a ground.

The device detects blood flow in the ascending aorta through the use ofa pulsed Doppler transducer (31) which is positioned and manipulatedwithin a patient's suprasternal notch. Preferably, the transducer probe(31) is of a commonly used type such as one having a single fixed focusbeam emitted from a transducer head. As shown in FIGS. 8 and 9, thetransducer probe preferably has a point of focus at a distance (d) ofbetween 7 and 10 cm from the transducer head. The transducer headpreferably comprises a single crystal for transmitting and receivingultrasonic energy signals.

A schematic diagram of the device is shown in FIG. 2. The major systemcomponents include a pulsed Doppler apparatus (2), a digital signalprocessor such as the TMS320C25 (Texas Instruments) (6), and amicroprocessor (8) such as the model 8088 (Intel Corp.) or V20 (NECCorp). The information processing and system control may be done by the8088 microprocessor or by a similar device. These components are readilyavailable, standard computer parts whose functions will be discussed indetail in the pages to follow.

Operational Modes

The device of the invention utilizes two operational modes during whichthe Doppler is active. They are (1) "search in depth" mode and (2)"acquisition" mode. During the "search in depth" mode, the device variesthe gate depth while taking data. At each gate depth, the usermanipulates the probe in the suprasternal notch of a patient in searchof a good signal. At the same time, the device performs real timepattern recognition of the received signals until twelve heart cycleshave been observed that pass the pattern recognition criteria. During"acquisition" mode, the device holds the depth constant and the useragain manipulates the probe in the suprasternal notch of a patient untilthe device collects twelve heart cycles that pass the patternrecognition criteria.

FIGS. 5, 6 and 7 diagram the processes of the device in its twooperational modes. In these figures, "D" generally refers to "search indepth" mode, and "E" refers to "acquisition" mode.

System Operation

In order to operate the cardiac output monitor typified herein, thedevice is connected to a power source. The date and time will appear onthe visual display indicating that the device is ready for use. The"start/yes" button is pushed to start the examination to which thedevice responds by displaying "Patient XXXXXXX".

The patient number is then entered using the key pad on the front panel.If an error is made, "stop/no" is pressed to clear the number and thenthe number may be entered again. When the correct number is displayed,the "start/yes" button is pressed to enter the number into the computer.

If a trial has been completed using the patient number just entered,"Use stored data?" will appear on the screen inquiring if theexamination should be taken using data stored previously. If the height,weight, and age of the patient have not changed, "yes" is pressed. Next"Print Data?" will appear. Answering "yes" will cause the device toprint the results of previous trials.

If the height, weight or age of the patient have changed, or if thepatient number is new, the computer will then ask for height, weight,and age of the patient and these values should be entered using the keypad. The "start/yes" button must be depressed after each number isentered to continue the program. If unreasonable data is entered, (e.g.,5'12"), "Input out of range" will appear. Pressing "start/yes" willremove the message and repeat the previous question. Pressing "stop/no"will abort the current trial and time and date will reappear on thevisual display.

Cardiac output can be measured on patients known to have an unusualaortic diameter by the following method. When "aortic diameter? y/n"appears on the display, "yes" is pressed. The display then requests theaortic diameter in centimeters. The measurement is entered to onedecimal place. If the aortic diameter is not known, "no" is pressedafter this question, causing the device to estimate aortic diameterusing the height, weight and age values that were entered at thebeginning of the examination.

If the patient has a thermodilution catheter in place, the aorticdiameter can be determined by the device using a back calculation. When"cardiac output? y/n" appears on the display, "start/yes" is pressed toinvoke the option. The display will request cardiac output in liters perminute. The current thermodilution cardiac output is then entered.

In the preferred system being described, all manually entered input mustbe within the ranges specified below:

    ______________________________________                                        Patient number        0 to 9,999,999                                          Height                                                                        (feet)                4 to 7                                                  (inches)              0 to 11                                                 Weight (pounds)       50 to 400                                               Age (years)           0 to 120                                                Aortic diameter (cm)  1.0 to 5.0                                              Cardiac output (1/min)                                                                              0.5 to 12.0                                             ______________________________________                                    

Entered values are stored in non-volatile memory (12). At this time, theprompts "Place probe . . . " and "Push switch to start" will appear onthe display. The patient is placed in a supine position, and a generousamount of ultrasound gel is applied to the probe face. The probe isgripped similar to the way a pencil is gripped for use and it is placedin the patient's suprasternal notch at an angle of approximately 90degrees with respect to the sternum, with the probe face pointinginferiorly. The probe is pressed firmly enough to allow the probe faceto protrude behind the sternum. The examination is started by pressingthe probe switch (16). A 7 or an 8 and "Find peak signal" will appear onthe display, indicating the depth in cm that the device is looking atfor a return signal. "Search in depth" mode has now been activated.

The probe angle with respect to the sternum is slowly maneuvered in therange of 30 to 90 degrees, while the operator observes the display. Thebest signal is found when the following occurs in conjunction with thedisplay of an "S": the bar graph of velocities extends as far aspossible to the right, there is little or no noise visible on thebargraph during diastole and systolic flow sounds emitted from the audiospeaker are as high pitched as possible. A poor signal can often beimproved by pressing in further with the probe to move the transducerhead inwardly of the sternum. When the operator determines that the bestsignal has been located, the probe is then held still and the probeswitch pressed, causing the device to collect 12 heart cycles suitablefor calculation of cardiac output.

Before the operator presses the probe switch, a "?" is shown to theright of the depth on the display. After pressing the probe switch, the"?" changes to a ">". The pattern recognition system is now active, andwill display an "S" in the far right hand segment of the display foreach heart beat for which the AVP passes the acceptance criteria. If the"S" never or rarely appears, the operator must reposition the probe towhere the "S" appears as frequently as possible.

The device will search through 7, 8, 9 and 10 cm depths testing for thebest depth at which to measure cardiac output. If the device finds noacceptable flow at any depth, this condition is displayed and the "usersearch mode" is reactivated. Otherwise, the "acquisition" mode isactivated at the already determined best depth.

During the first six seconds of "acquisition mode", a ")" is shown tothe right of depth on the display. During this time, the automaticpattern recognition of good signals is not active. After six seconds, a">" replaces the ")", indicating the pattern recognition has becomeactive. The initial six seconds are meant as a time for the operator tore-locate the best signal by manipulating the probe.

If the pattern recognition does not complete a trial within severaldozen heart cycles, whether during "search in depth" or "acquisition"mode, the operator may terminate the trial at that depth by depressingthe probe switch for a period greater than 0.25 seconds. Doing thismaneuver during "search in depth" mode will cause the device to jump tothe next depth. During "acquisition" mode, the device will ask theoperator whether or not to repeat the "search in depth". If the operatordoes not choose to repeat the "search in depth", the device aborts thecurrent examination displays the date and time.

Cardiac output and heart rate are displayed after the successfulcompletion of an "acquisition" mode. Pressing the probe switch at thistime causes the device to re-execute the "acquisition" mode. Pressing a"yes" at this time allows the operator to print results and end theexam. Pressing "no" aborts the current study and the device displays thedate and time.

The power switch on the back panel of the housing should only be turnedoff when the device is displaying the date and time. If it is desired toturn the system power off when the date and time are not displayed,press "stop/no" and the date and time will appear.

If results are obtained, the device may first display either or both oftwo warnings before displaying the results. The first warning is"Warning: reverse flow", and indicates that significant flow wasdetected in the reverse flow direction. The second warning is "Warning:High veloc", and indicates an abnormally high flow velocity detected forthis patient. The warnings are meant to suggest the presence of apathological abnormality or noisy signal for which the calculatedcardiac output might be unreliable.

Up to five tests may be stored for each patient number. When a sixth orsubsequent test is stored, the oldest test is deleted from memory. Aprinted report is obtained by pressing "yes" in response to "Testcomplete-print". This report will contain the previous 1 to 4 storedtrials and the current trial. To do another test on the same patient,"yes" is pressed in response to "Repeat test? y/n". The device thenreturns to "acquisition" mode. If an hour or more elapses before doing atrial again on the same patient, an abbreviated form of the "search indepth" mode is performed at the depths nearest the already chosenoptimal depth. At this time, signal characteristics are re-assessed anda new optimal depth is chosen.

Description of Pulse Doppler Receiver

The Doppler receiver can be divided into two RF stages and five audiostages. The RF-preamp and inter channel isolation network (emitterfollowers) comprise the two RF stages. The five audio sections are: thedemodulator with its low pass filter; the sample-and-hold with lowfrequency canceller; the smoothing filter; the programmable band passfilter; and finally the quadrature balance stage. All stages have gain.

The RF-preamp is a fairly broad band amplifier, from 1.5 Mhz to 5 Mhz.At 2.4 Mhz, the gain of the RF-preamp can vary from 18 to 52 dB in 255steps. The demodulator circuit has 20 dB of conversion gain. Thedemodulator low pass filter has a break frequency at 155 khz with a passband gain of 18 dB.

The sample-and-hold section following the demodulator uses feedbackcancellation of signals below 500 Hz which represent artery wall motion.The overall effective passband of this section is 500 to 4,000 Hz.

The smoothing filter on the output of the sample-and-hold circuit has alow pass response. The corner frequency is 10 khz. The corner frequencyis adequate for switching transients of the sample-and-hold transitions.The smoothing filter has 18 dB of gain over its pass band.

The programmable filters have an out of band rejection of at least 50dB. The filter is set for 0 db in the pass band. The signal pass band isset at 550 to 5.0 khz. The corner can be adjusted as needed.

The final stage, before the A/D converter, is a quadrature balancecircuit. This stage allows manual trim of the balance between the twoquadrature channels and has 14 dB gain from 337 Hz to 9.5 kHz.

Microprocessor Algorithm Components

Four terms are used frequently in referring to the software of thedevice. The "envelope" is a collection of 125 blood velocity valuescalculated per second by the digital signal processor. The "firstmoment" is the sum of a magnitude-frequency product: [i*f(i)], wheref(i) is the ith magnitude determined from the 128 point complex FFT (seesection entitled "Digital Signal Processor Software Description" below).The "first moment" (FM) is obtained by the formula FM=SUM[i=b1,N](i*f(i)), where N indicates the number of positive magnitudes ofinterest. FM is a function that remains close to zero during diastoleand rises steeply away from zero during systole. The "first momentderivative" is a value which represents the amount of change per unittime of the averaged first moment values.

The energy of forward flow is calculated by the device using thefollowing formula: EFF=SUM[i=1,N] (f(i)*f(i)), where EFF is the energyof forward flow in units of volts, N is the number of positivemagnitudes, and f(i) is the ith spectral coefficient.

Forward flow is determined to be occurring if the first moment exceedsthe value T1 and the energy of forward flow exceeds the value T2. T1 andT2 are thresholds that are later defined in equations 1.3 and 1.4,respectively, and are updated at the end of every two second period. Ifforward flow is determined not to be occurring, then the value of zerois given to the microprocessor as the value of the envelope. Otherwise,a sum is begun from the highest frequency of spectral energy. Thespectral energy for the next lowest frequency is added to the sum untilthe sum exceeds one-fourth of the total spectral energy. The frequencyat which this occurs is given to the microprocessor as the value of theenvelope.

The envelope, the first moment, the averaged first moment, and thederivative of the averaged first moment are each calculated 125 timesper second. These values will be hereafter respectively referred to asIENVEL, FM, CAVER, and DERIV. These values are stored in arrays overtime, so the most recent 256 values of each are always available. Thus,when adding an index to these variable names, we are referencing thevalue that occurred at a particular time, e.g., IENVEL[i] indicates theith value of the envelope, FM[i-2] indicates the (i-2)th value of thefirst moment, etc.

The primary components of the microprocessor algorithm determine thefollowing values or events: threshold used in determining the start ofsystole, the start of systole, whether an AVP is acceptable, the firstmoment average and its derivative, the real time bargraph, the selectionof optimal depth, heart rate, correlation of velocity signals fromdifferent heart cycles to produce one representative envelope,calculation of aortic diameter and body surface area, and calculation ofthe values of cardiac output, cardiac index, stroke index, maximumvelocity, peak acceleration, and ejection time.

These algorithm components can be separated into those jobs done in realtime, i.e., while the transducer and the digital signal processor areboth active, and those jobs not done in real time, i.e., when thetransducer is inactive.

A. REAL TIME JOBS 1. Thresholds Calculated in the MicroprocessorSoftware for Use in Determining Systolic Flags

There are two thresholds in the microprocessor software that arefundamentally involved in detecting the occurrence of a systole. DMAX isa value the derivative of the averaged first moment must exceed forrecognition of a systolic event, and CMIN is a value that must beexceeded for recognition of a systolic event. It is necessary to haveinformation in these thresholds that spans more than two seconds if aminimal number of false systoles are to be detected. False systoles cancome from noise spikes during a time of little signal, such as whenbreath motion or searching with the probe takes the field of view awayfrom the aortic signal.

The methods for finding CMIN and DMAX are essentially the same. First,the running maximum of the first moment, FMmax, and running maximumfirst moment derivative, DERIVmax, are calculated by keeping track ofthe maximums of FM and DERIV across 256 iterations, roughly two seconds.Also denote CMIN' and DMAX as the most recent values of CMIN and DMAX.The next values of CMIN and DMAX are calculated by the followingequations: ##EQU1##

The effect of this method is that wall thumps and other large energynon-systolic signals (temporarily seen) by the device will not cause thethresholds for detecting systoles to become suddenly too high.

2. Calculation of First Moment Average and Its Derivative

The first moment average, CAVER, is calculated before taking the firstmoment derivative, DERIV. CAVER is a six point moving average of FMstarting with the current iteration and including the five next previousiterations. The derivative is then calculated by the rule:DERIV[i]=(CAVER[i]+CAVER[i-2]-CAVER[i-12]-CAVER[i-14])/2. Here thederivative need not be normalized.

3. Determination of a Sysolic Chart

The following are six criteria that must be met for a systolic start tobe recognized by the device (note this is different than the task ofdiscriminating whether an occurred systole can be used for calculationof cardiac output):

a) The first moment, FM, has reached a minimum steepness, DMAX.

b) 240 ms ago, the envelope IENVEL depicts an absence of flow.

c) The current envelope depicts flow.

d) The derivative of the first moment average, DERIV, has reached alocal maximum.

e) The previous systole occurred more than 400 ms ago.

f) The first moment, FM, has exceeded a minimum value, CMIN.

4. Barograph Algorithm

Preferably, the bargraph displays a visual representation of a signalcorresponding to blood flow velocity from 0.0 to 4.5 kHz on any of avariety of 20 character alpha numeric displays whose characters are madeup from 5 by 7 dot matrix elements. Any signal above 4.5 kHz isdisplayed as 4.5 kHz.

In a preferred embodiment, the display contains a total of twentycharacters, sixteen of which are used for the bargraph, two for displayof depth (leftmost two characters) one for a bargraph active indicator(">", ")", or "?" to the right of depth), and one for a "good" systoleindicator (an "S" at the far right box). The instantaneous modalfrequency is displayed by a row of solidly lit 5×7 characterscorresponding to increasing frequency as characters are lit from left toright. Referring to the bargraph active indicators, a "?" indicates thedevice is searching in depth for the best signal, a ")" indicates datais not presently being acquired, and a ">" indicates data is presentlybeing acquired. Signals above 4.5 kHz, corresponding to high bloodvelocities, are clipped to the upper end of the bargraph (e.g., box #18of 0-19) so that signals corresponding to velocities below 1.5 m/s aregiven the greatest attention.

The device additionally displays persistently a row of colons residingin the "background" of the solid characters. This row of colonsrepresents the average peak systolic velocity for the preceding foursystoles. The peak velocity for a given systole is determined by findingthe maximum "envelcpe" value during a 100 ms time span, whose center isthe systolic indicator. If no systole occurs for approximately threeseconds, then the row of colons is cleared and the four beat average isrestarted.

5. Pattern Recognition of Signals Suitable for Calculation of CardiacOutput

There are three parallel paths comprising the pattern recognitionalgorithm of the preferred embodiment. Path #1, which exists in thedigital signal processor, is shown in FIG. 10 and described in thesection entitled DIGITAL SIGNAL PROCESSOR SOFTWARE DESCRIPTION. Paths #2and #3 exist in the microprocessor and are shown in FIG. 10A.

Path #2 is used to determine if the velocity profile following the mostrecent systolic flag should be included in the calculation of cardiacoutput. This decision must be made during the 320 ms following thedetection of a systolic event (see section #3) The decision isaffirmative if two conditions are simultaneously true at some pointduring the 320 ms window. The first condition is that the normalizedKarhunen Loeve parameters K1 and K2, calculated in the DSP, obey thefollowing rule: ##EQU2## The second condition is that the maximum valueover the past 400 ms of the median filtered velocity profile, h, isgreater than Vd. Vd, the "desired velocity" to be observed, is a valuederived during search mode. The calculation of Vd is described in thesection entitled "DETERMINATION OF THE DEPTH CONTAINING THE OPTIMALSIGNAL." Generally when an operator moves away from the signal in thecenter of flow, the maximum velocity, h, will decrease. The secondcondition thus acts to keep the probe trained on the best possiblesignal.

If the observed velocity profile satisfies the conditions of path #2,then the systole is loaded into the top of a "systole buffer," and allother systoles are displaced one step backward in the same buffer. Iftwelve systoles exist in the systole buffer, then path #3 determines (1)if the standard deviations of maximum velocity and stroke distanceacross the buffer are respectively within 20% and 25% of their means,and (2) if the automatic gain control has not been steadily rising orfalling during the past six seconds. The effect of path #3 is to notallow the exam to be completed unless the group of twelve observedsignals can be said to contain a certain resemblance to each other. Ifthe path #3 conditions are true, then the exam is concluded and thecardiac output and related parameters are determined.

(a) DERIVATION OF EQUATION 5.1

Equation 5.1 was found through statistical analysis of two independentgroups of aortic velocity profiles. The first group of AVPs was used todetermine an efficient way to represent the signal class of all AVPs, byuse of the discrete Karhunen Loeve (KL) expansion. The derived KLexpansion was applied to the second group of AVPs. On the second groupof AVPs, a pattern recognition boundary based on two KL coefficients wasfound that separated signals suitable for calculation of cardiac outputfrom signals not suitable for calculation of cardiac output. Thispattern recognition boundary (Equation 5.1) was chosen to yield a 16%false alarm probability, and results in a detection probability of 0.84.

(i) THE FIRST GROUP OF AVPs: Applying the Karhunen-Loeve Expansion

The first group of AVPs was assembled from the averaged results of eachof 234 studies on both normals and intensive care patients. From thisset of average AVPs, 164 were selected which fell well within the rangeof signals characteristic to the ascending aorta.

The set of 164 averaged AVPs forms the signal class [X(i): i=1,164].Each AVP contains 50 discrete values and a sample period of 8.0 ms,yielding a total length of 400 ms. The average of all signals in [X] isshown in FIG. 11, and is denoted by M. The discrete Karhunen-Loeveexpansion was applied to [X] to determine a small set of K orthogonaleigenvectors, [p(i): i=1 . . . K], defining the space containing thepredominant signal class energy.

[p] was derived by first constructing the zero mean set [X':X'(i)=X(i)-M, i=1 . . . 164]. The covariance or average outer productmatrix was formed by the rule

    G(j,k)=E]X'(i,j),X'(i,k)], j,k=[1,50],

where X'(i,j) is the jth value of the ith signal X'(i).

G was decomposed into fifty eigenvectors [P(i): i=1,50] and eigenvalues[L(i): i=1,50], where P(i) and L(i) are ordered from greatest eigenvalue(L(1)) to least eigenvalue (L(50)). L(i) is proportional to the energyof the signal class (X') in the direction of the corresponding itheigenvector.

The set [p] consists of the first K vectors of the set [P]. Inparticular, for any fraction of the total signal class energy, there isa minimum value of K such that the predominant energy of the signalclass [X'] is still contained in [p]. The relative values of the four(K=4) largest eigenvalues of [L] are shown in FIG. 12, and thecorresponding eigenvectors p(1) . . . p(4) are shown in FIG. 13. Thesefour eigenvectors together span a space containing 90% of the signalclass energy. Any AVP, A, can be efficiently represented by the valuesK1, K2, K3, and K4:

K1=<A-M, p(1)>

K2=<A-M, p(2)>

K3=<A-M, p(3)>

and K3=<A-M, p(3)>.

NOTE: The notation <a,b> denotes the dot product between the vectors aand b.

(ii) THE SECOND GROUP OF AVPs: Deriving a Pattern Boundary

The second group of AVPs, [Y], was assembled from over 3300 non-averagedaortic velocity profiles sampled from both normals and intensive carepatients. [Y] was divided by experts into signals suitable forcalculation of cardiac output, [YG], and signals not suitable forcalculation of cardiac output, [YB]. AVPs in [YB] are typicallycorrupted by noise, or contain an abnormally short systolic interval.[YG] contains 2445 AVPs, and [YB] contains 867 AVPs.

Experiments showed that K1 and K2 are clearly important in separation of[YG] from [YB], but only after pre multiplying each signal Y(i) by thefactor Nu, where Nu=max(M)/max(Y(i)). K1 and K2 were determined for eachsignal Y(i) after multiplying by Nu. A modified simplex search algorithmwas then used to find the linear pattern boundary, K2=f(K1), whichoptimizes the detection of [YG] without allowing more than 16% falsealarms from [YB]. The optimized detection probability of [YG] is 0.84,and the resulting pattern boundary is Equation 5.1.

Reference for modified simplex search algorithm: [Reklaitis, et al.,Engineering Optimization. Wiley and Sons, New York: 1983, pp. 76 ff.]

(b) NOTE ON THE REAL TIME IMPLEMENTATION OF EQUATION 5.1

Equation 5.1 was constructed from the AVP groups [X] and [Y]. The leftedge of every AVP in [X] and [Y] was aligned at 40 ms before anyanalysis was performed. In real time, the window containing the "currentsignal" is moving in time, and rarely is the left edge of systolelocated at 40 ms into this window. Equation 5.1 is still valid withoutany alignment of the real time AVP if two conditions are observed. Thefirst condition is that given the AVP A, the factor Nu is modified inthe following way: ##EQU3## This condition is implemented in the digitalsignal processor (see DITIGAL SIGNAL PROCESSOR SOFTWARE DESCRIPTION andFIG. 10). The second condition is that Equation 5.1 can only be appliedto accept or reject an AVP during the 320 ms following a systolic start.This condition is implemented in the microprocessor.

B. JOBS NOT DONE IN REAL TIME 1. Determination of the Depth Containingthe Optimal Signal

Twelve systoles are sampled at each depth during the "search in depth"mode. This set of aortic velocity profiles is denoted by B=(b1,b2, . . ., b12). The maximum average velocity for a given depth, Vmax, iscalculated as follows:

(a) Median filter and zero the diastolic section on each of the profilesof B as described later in parts (a) and (b) of the CORRELATION AND DATAREJECTION ALGORITHM below.

(b) For each profile, bi, find the maximum value of a "reconstructed"profile, Vi, according to the following formula:

    Vi=maximum [M+K1*E1+K2*E2+K3*E3+K4*E4]

where M is the mean aortic velocity profile E1 . . . E4 are theKarhunen-Loeve eigenvectors with the four largest eigenvalues

K1=<bi-M,E1>

K2=<bi-M,E2>

K3=<bi-M,E3>

K4=<bi-M,E4>

NOTE: The notation <a,b> denotes the dot product between the vectors aand b.

(c) Now with the set of reconstructed maxima, (v1,v2, . . . ,v12), findthe average value:

Vmax=mean(v1,v2, . . . ,v12).

The optimal depth is typically the depth whose Vmax is largest, unlessVmax for seven cm is the largest and Vmax for eight cm is the least. Inthat case, it is assumed that the innominate artery is actually at sevencm and the aorta is deeper. Under this condition, nine or ten cm ischosen as the depth of interest, whichever produced the highest value ofVmax.

Now the "desired velocity" for this optimal depth, Vd, is calculated as85% of Vmax. For future measurements of cardiac output, the operatorwill be required to locate signals whose maxima, after median filteringand zeroing the diastolic sections, exceed Vd.

2. Determination of Heart Rate

Heart rate is calculated from the modal average

systolic flags. Since twelve heartbeats are collected during the dataacquisition mode, there are eleven resulting values of "time betweensystolic flags". Call these values D=[d1,d2, . . . ,d11]. A value dm isdefined as the value in the D which has the greatest number of remainingvalues in D falling within +/-20% of its own magnitude. If the number ofvalues falling within this +/-20% range of dm does not exceed five, thenthe examination is rejected due to a lack of confidence in the heartrate calculation. If the number of values falling in the +/-20% range ofdm exceeds five, then the mean average of this group, including dm, isreported as the time between systoles. The reciprocal of this reportedvalue is the heart rate in beats per minute.

3. Correlation and Data Rejection Algorithm

When a "good" systole occurs (see section #5 of JOBS DONE IN REAL TIME),the envelope IENVEL across the duration of that systole is stored inmemory. The stored IENVEL values for one systole have been previouslydefined as an AVP. There are four steps involved in taking these storedsystolic profiles for twelve heartbeats and producing one representativesystolic profile:

a) Median filter all stored AVPs to remove non-physiological variationsin apparent AVPs. Define b1 to be a stored AVP. When this stage of thealgorithm is reached, there exists a set of twelve AVPs: (b1,b2, . . .,b12). b1[i] is the ith in time value of b1. The median filter appliedto each value of the stored AVP b1 is defined as:

    a=(b1(i-2)+b1(i-1)+bl(i+1)+b1(i+2))/4

if abs(a-bl(i))>21 cm/s then b1(i)=a

This filter is performed so that the unfiltered values b1(i) and b1(i-1)are used in filtering b1(i+1).

b) Zero the diastolic portion of each stored AVP. On each AVP firstlocate the time of the ascending portion that is 32 ms or 4 iterationsbefore the 20% of maximum velocity point. Call this kstart. Similarly,locate the time of the descending portion that is 32 ms or 4 iterationsafter the 25% of maximum velocity point. Call this kstop. Then zero allvalues before kstart and after kstop.

c) Correlate and average the twelve stored systoles. If x, y and zdenote stored systolic profiles, then let the cross-correlation functionx=C(y,z) denote the operation of aligning y and z via a maximum in thecross correlation function and storing the resulting average systolicprofile to x. The following recursive sequence of operations isperformed:

x=C(b1,b2)

x=C(x,b3)

x=C(x,b4)

x=C(x,b5)

x=C(x,b6)

x=C(x,b7)

x=C(x,b8)

x=C(x,b9)

x=C(x,b10)

x=C(x,b11)

x=C(x,b12)

d) Sum the elements stored in x and divide by 12 to yield the averagecorrelated systolic velocity profile.

4. Determination of Aortic Diameter and Body Surface Area

A paper titled, "Nomogram for Determining Normal Aortic Diameter (AorticArch) and Aortic Biological Age in 2-m Chest X-Rays" by Strehler, etal., CIBA-GEIGY Limited, CH-4055 Basle, Switzerland was used as areference for determining a formula for body surface area. An analysisof this paper was performed regarding calculation of aortic diameter incomparison with m-mode root aortic measurements done at ProvidenceMedical Center, and a linear correction to the CIBA-GEIGY results wasadded.

The formula used to calculate body surface area is:

    BSA=exp(log(L)+log(G)/3.0 log(3.85))/100.0.

In this formula, G is weight (in kilograms), and L is height (in cm).Aortic diameter is found using the following formula:

    adiam=0.98*gamma+0.316

where

    gamma=exp(exp(alpha))*1.12-0.3

    alpha=log(BSA)/2.0+log(log(A))-log(5.15). A is age (in years)

5. FORMULAS FOR FINAL VALUES

    CO=SV*HR/1000. (liters/min)

where

HR=Heart rate, algorithm described separately.

SV=SD*3.1415926*adiam*adiam/4 (cc)

SD=Stroke distance m/s (Trapezoidal integration of the flow velocityprofile over the systolic period from t0 to t1) cm/s.

t0=Leading edge of systole determined by going backward in time from thepeak to the 2nd profile element below 20% of the peak or the first 0point below 20% of the peak, whichever occurs first. Each profileelement is spaced by 8.005 mx in time.

t1=Lagging edge of systole determined by going forward in time from thepeak to the 2nd profile element below 25% of the peak.

adiam=aortic diameter, cm.

CI=Cardiac Index (liters/min/sq meter)=CO/BSA where

BSA=Body surface area, sq meters.

Stroke Index=SV/BSA. (cc/sq meter)

Maximum Velocity=Maximum value of flow velocity profile, cm/s.

Accleration=Peak instantaneous acceleration value, meters/sq second, ascalculated with the following algorithm and 16 point finite impulseresponse (FIR) filter:

ALGORITHM FOR CALCULATION OF ACCELERATION

(a) Find i such that velocity [i+1]-velocity[i] is maximum for wholeprofile.

(b) At j=i and j=i+1 perform the 3 point average velocity[j]=(velocity[j+1]+velocity[j]+velocity[j-1])/3

(c) Apply derivative filter to velocity[]. ##EQU4##

(d) Find maximum in resulting derivative.

    Ejection Time=t1--t0 (ms)

Digital Signal Processor Hardware Description

The digital signal processing section of the device comprises a signalprocessor, an A/D buffer memory, and a program and data transfer memory.The A/D buffer memory is 1024 words long, each word being 12 bits wide.This buffer is a switched type double buffer such that the A/D and thedigital signal processor both share the address and data lines. Theprogram memory and data transfer memory is 2048 words long, each wordbeing 16 bits wide. The digital signal processor code operates from thismemory. It is preloaded with the program by the microprocessor. Thisbuffer is a switched type double buffer such that the digital signalprocessor and the microprocessor both share the address and data lines.

Digital Signal Processor Software Description

The digital signal processor code allows 125 sets of features to becalculated per second. The Doppler shift signals are input from the A/Dconverters, a spectra is calculated, features are extracted from thespectral coefficients and the results are output to the microprocessor.

The input to the digital signal processor is from a circular bufferimplemented in hardware. This buffer is 1024 12-bit words long. It isfilled with quadrature data from the A/D converter, alternating in-phaseand out-of-phase values. The 256 words are read from this buffer intothe digital signal processor 125 times per second. These 256 wordsrepresent 128 complex pairs. Each set of 256 words overlaps. Errorchecking occurs to insure that the digital signal processor issynchronized with the A/D state machine.

Once these 256 words are input to the digital signal processor, they aremultiplied by a Hamming Window. This window is used to reduce theeffects of boundary condition violations. Next, a 128-point complex FFTis performed on the data. The magnitudes are then calculated from thespectral coefficients using a Taylor series expansion. The expansionperforms the calculation in either Equation 1.1 or 1.2 based on which ofthe two coefficients, the real or imaginary, are greater. Since thedigital signal processor is an integer processor, a portion of theinteger is dedicated to fractional positions. ##EQU5##

The resultant 128 magnitudes are then thresholded such that anymagnitude falling below a threshold is set to zero. The threshold is setas a constant at the start of the program. In addition, the maximummagnitude is determined.

The next steps involve calculation of the features. The features includethe positive flow energy, negative flow energy, first moment, envelopevalue, Karhunen-Loeve parameters, envelope maximum velocity after medianfiltering (see section #3 for median filter rule), and envelope integralafter median filtering (stroke distance). The positive flow energy isobtained by summing the squares of the magnitudes for coefficients from1 to N where N is the number of positive flow coefficients. The negativeflow energy is obtained by summing the squares of the magnitudes for thecoefficients from N+1 to 128.

A set of running globals are also calculated each iteration. Theseinclude the running spectral first moment maximum and minimum, and therunning energy maximum and minimum. The running maxima and minima arethe maxima and minima relative to the current set of 256 iterations.Every 256 iterations, the values "global first moment threshold" and"global energy threshold" are calculated based on the running maxima andminima of spectral first moment and energy.

From the set of 128 magnitudes from one FFT, the first moment (FM) iscalculated by summing the result of multiplying each magnitude by thebin number of that magnitude. The envelope (iENVEL) is determined bycomparing the integral of the total energy counting top down through thepositive magnitude coefficients until the first bin that has anaccumulated energy greater than the energy threshold. The energythreshold is equal to the total energy divided by four. The envelope isset to zero if FM is less than the global first moment threshold or ifthe positive energy is less than the global energy threshold.

The running first moment maximum and minimum and the running energymaximum and minimum are calculated by keeping track of the maxima andminima of these values across 256 iterations, roughly two seconds. Therunning first moment maximum(minimum) is updated if the local firstmoment is greater(less) than the running first moment maximum(minimum).The running maximum energy is calculated by adding an energy incrementif the total energy is greater than the running maximum energy. Thisenergy increment restricts the rate at which the running maximum energycan rise. The running minimum energy is set to the local total energy ifthe local total energy is less than the running minimum energy.

The global first moment threshold and the global energy threshold aredetermined by Equations 1.3 and 1.4, respectively: ##EQU6##

FIG. 10 shows the DSP path for calculation of the NormalizedKarhunen-Loeve parameters Kx and Ky, as well as the stroke volume SV andthe maximum velocity h. This flow, noted as Path #1 in this document, isexecuted every 8 ms. Thus for every calculation of an FFT and aresulting IENVEL value, there is an accompanying calculation of[Kx,Ky,SV,h].

Let S denote the velocity profile spanning the most recent 400 ms. LetS1 denote the resulting profile after median filtering S and zeroing itsdiastolic sections (see parts (a) and (b) of the CORRELATION AND DATAREJECTION ALGORITHM). Then h is simply the maximum value of S1, SV isthe sum of all 50 values contained in S1, and Kx and Ky are found asfollows: ##EQU7##

Once the features are calculated, the microprocessor is signalled,indicating that the digital signal processor is done with another cycle.The microprocessor then puts the digital signal processor in a holdstate, reads the features from the program memory, and releases thedigital signal processor.

While a preferred embodiment of the present invention has beendescribed, it should be understood that various changes, adaptations andmodifications may be made therein without departing from the spirit ofthe invention and the scope of the appended claims.

What is claimed is:
 1. A cardiac output measuring device for detectingblood flow in a patient's ascending aorta, comprising,an ultrasonictransducer probe adapted to be received in the patient's suprasternalnotch, including means for generating ultrasonic energy pulsesdirectable toward the ascending aorta when the probe is positioned inthe suprasternal notch, and receiving reflected frequency shifted energypulses having a frequency shift related to the velocity of blood flowthrough the ascending aorta of a patient, electronic means forseparating the reflected frequency shifted signals from non-frequencyshifted signals to form Doppler shift signals, and demodulating theDoppler shift signals into the normal audio frequency range, computingmeans responsive to said Doppler-shift signals for computing the cardiacoutput of the patient, means for selecting the depth within the aorta ofthe patient at which the velocity of blood flow is measured, said meanscomprising means for varying the time interval between the time at whicha signal is transmitted and the time the reflected signal received bythe ultrasonic transducer probe is selected for processing, the timeintervals corresponding to varying distances from the probe andtherefore also to linear positions within the ascending aorta, patternrecognition means for testing a time series of received Doppler shiftsignals against predetermined signal quality characteristics and foraccepting such signals as meet such characteristics, and meansresponsive to said accepted signals for choosing the linear positionwithin the ascending aorta corresponding to the greatest frequencyshift.
 2. The cardiac output measuring device of claim 1 including dataentry means for entering height, weight and age data for thepatient,means for estimating aortic diameter from such height, weightand age data, means for calculating heart rate using said acceptedsignals, and said computing means responsive to said estimated aorticdiameter and the accepted signals for computing cardiac output.
 3. Thecardiac output measuring device of claim 2 wherein the manual data entrymeans comprises an electronic keypad.
 4. The cardiac output measuringdevice of claim 1 including manual data entry means for detecting aorticdiameter data obtained from external measurements, and means responsiveto the aortic diameter obtained from external measurements and theaccepted frequency shifted signals for computing cardiac output.
 5. Thecardiac output measuring device of claim 1 wherein the means forgenerating and receiving ultrasonic energy pulses comprises at least onepiezoelectric crystal.
 6. The cardiac output measuring device of claim 1including means for generating an audio signal having a frequencycorresponding to the blood flow velocity calculated in the ascendingaorta.
 7. The cardiac output measuring device of claim 1 wherein thepattern recognition means for testing a time series of receivedDoppler-shift signals includes means for calculating Karhunen-Loeveexpansion coefficients in real time and means for forming a primaryfeature set of signal characteristics using the Karhunen-Loeve expansioncoefficients.
 8. The cardiac output measuring device of claim 1 whereinthe pattern recognition means for testing the time series of receivedDoppler-shift signals comprises:means for calculating in real time a setof normalized Karhunen-Loeve expansion coefficients of the time seriesof received Doppler-shift signals; means for calculating in real time astroke distance and a peak velocity of the time series of receivedDoppler-shift signals; means in real time for determining from thenormalized Karhunen-Loeve expansion coefficients whether the time seriesof received Doppler-shift signals is suitable for calculation of cardiacoutput; means for collecting a plurality of the suitable time seriesfrom which to calculate cardiac output; means for computing in real timea standard deviation and means of the suitable stroke distances, and aratio of the standard deviation to the mean; means for computing in realtime a standard deviation and mean of the suitable peak velocities, anda ratio of the standard deviation to the mean; and means in real timefor determining from the ratio of the standard deviation of the suitablepeak velocities to the mean of the suitable peak velocities and from theratio of the standard deviation of the suitable stroke distances to amean of the suitable stroke distances, whether or not to conclude thecollecting of the suitable time series, and whether or not to report thecardiac output measurement.
 9. A cardiac output measuring device fordetecting blood flow in a patient's ascending aorta, comprising(a) anultrasonic transducer probe having a head adapted for insertion in apatient's suprasternal notch, (b) means for generating ultrasonic energypulses and receiving a time series of reflected frequency shifted energypulses having a frequency shift proportional to the velocity of bloodflowing through the ascending aorta, (c) means for receiving, andamplifying the reflected frequency shifted pulses, (d) means fordetermining from the magnitude and the direction of the shift frequency,the corresponding velocity of blood flow, (e) pattern recognition meansfor selection of reflected, Doppler frequency shift signals for use incalculating cardiac output, said pattern recognition means comprisingmeans for selecting and accumulating such Doppler shift signals asqualify under all of the following criteria; the first moment of andtime derivative of such Doppler shift signals indicates blood flowtoward the transducer probe in the forward direction, and for aspecified period of time preceding the reception of such Doppler shiftsignals, signals corresponding to blood flow in the direction toward thetransducer probe were not detected; (f) means for testing accumulatedtime series of Doppler shift signals against predetermined signalquality characteristics and for accepting and averaging such signals asmeet such characteristics, (g) calculating means for calculating heartrate from said accumulated time series of signals, (h) means forestimating aortic diameter from height, weight and age data, (i)computing means responsive to said averaged signal, said heart rate andaortic diameter for computing the cardiac output of the patient, and (j)validity testing means responsive to said heart rate and said averagedsignal for determining the validity of the cardiac output measurement.10. A cardiac output measuring device for detecting blood flow in apatient's ascending aorta comprising(a) an ultrasonic transducer probereceivable in a patient's suprasternal notch, the probe having means forgenerating ultrasonic energy pulses directable toward blood flowing inthe ascending aorta and receiving reflected frequency shifted energypulses representative of the velocity of blood flow through theascending aorta of a patient, said transducer having a handle adapted tobe manually held by an operator, and (b) visual display means responsiveto said received frequency shifted energy pulses for displaying a visualsignal representing the velocity of blood flow in the ascending aorta,the visual display signal corresponding to blood velocity of eachheartbeat being overlaid upon the signal for the preceding heartbeat onthe visual display means, and (c) peak indicator means responsive toeach of said blood velocity signals for producing a visual flag on saidvisual display means representing the peak value of each such signal,the peak flag for each signal remaining on the visual display means atleast until the next heartbeat peak so that it may be compared with thepeak flag position for the next succeeding signal.
 11. A method forderiving an efficient set of features with which to perform real timestatistical pattern recognition on time series of received Doppler-shiftsignals, comprising the steps of:(a) collecting a large group of timeseries Doppler-shift signals; (b) determining a set of Karhunen-Loeveexpansion eigenvectors from the large number of time series of receivedDoppler-shift signals; (c) collecting a second large group of timeseries Doppler-shift signals and separating said second group intopreferred signals and unpreferred signals; (d) calculatingKarhunen-Loeve coefficients using the Karhunen-Loeve expansioneigenvectors; (e) normalizing the Karhunen-Loeve coefficients; (f)determining which karhunen-Loeve coefficients are statistically relevantfor use in separating said preferred signals from said unpreferredsignals; and (g) determining a discriminant relation of saidstatistically relevant Karhunen-Loeve coefficients for use in separatingsaid preferred signals from said unpreferred signals.